You can never have too many amps! Nuro is correct however, that if you plan to run digital, you won't need more than 10 amps because you won't be running anything hotter than stock motors (because the decoders won't handle more). On the other hand, if you run analog and want to use TSRF or Falcon motors, for example, they can draw 3-4 amps each, so a 30 amp supply is not unreasonable on a 4-lane track.
My personal preference is for a supply that exceeds my calculated needs by 50% or more. That eliminates any possibility of surging and also keeps the supply operating well below its limits, extending the life of the supply to near infinity.
Astro, the 30-amp rating is the supply output current and not its input requirements. My supply, for example, can produce 6-amps at each of it's four independent outputs, but the input requirement is not more than 4 amps or so. When you reduce voltage, as a power supply does, the output current can be increased at roughly the same ratio (not quite because of inefficiencies but you get the idea).